This invention relates to capacitance measuring systems.
The invention is more particularly, but not exclusively, concerned with systems for measuring the value of capacitance fluid-gauging probes.
The height of fuel in an aircraft tank is commonly measured using capacitive probes. These take the form of concentric tubes separated from one another by an annular gap, which is filled with fuel to the same height as outside the probe. As the level of fuel rises or falls, the different dielectric properties of the fuel and air cause the capacitance of the probe to change. Most commonly, a balanced bridge circuit is used to measure the value of the probes. The bridge circuit has a reference capacitor, which is adjusted to minimize the output voltage. Alternatively, voltages applied across the two capacitors are adjusted to achieve balance. There are, however, problems in measuring the value of the capacitance probes in this way. The cable interconnecting the probes with the measuring circuit often has a capacitance exceeding that of the probes. Also, aircraft environments often have high levels of electrical noise. Furthermore, the potentially flammable gases above the fuel in the tank make it important to prevent stray currents entering the tank via the gauging system. This is usually overcome by means of zener diode barriers in the circuit but these increase impedance and increase sensitivity to parasitic cable capacitance. Phase delays can occur as a result of the combination of the zener barrier and cable capacitance.
It is an object of the present invention to provide an alternative capacitance measuring circuit.
According to a first aspect of the present invention there is provided a capacitance measuring circuit including a first capacitance to be measured, a second, reference capacitance, a first dc voltage source, a second dc voltage source and switching means operable to charge and discharge the first and second capacitances, the system being arranged such that charges of opposite senses from the first and second capacitances are summed and the outputs of the first and second voltage sources relative to one another are altered such as to minimize the sum of the charges, and the system being arranged to provide an output representative of the capacitance of the first capacitance in accordance with the relative values of the voltage sources.
The switching means preferably includes pairs of switches operated in antiphase with one another such that one connects a voltage source to a capacitance and the other connects a capacitance to ground. The circuit may include amplifier means and switching means operable to rectify and supply the summed charge to the amplifier means. The switching means preferably includes a pair of switches operated in antiphase, one being connected to the amplifier means and the other being connected to ground. An output from the amplifier means is preferably operable to control the relative values of the voltage sources.
According to a second aspect of the present invention there is provided a capacitance measuring circuit including first and second dc voltage sources having one output connected with a 0 volts rail and having their other outputs connected respectively to a capacitance to be measured and to a reference capacitance via respective first and second switches operated in anti-phase with one another, the circuit including a third and fourth switch each connected between the 0 volts rail and respectively the capacitance to be measured and the reference capacitance, the third switch being operated in anti-phase with the first switch and the fourth switch being operated in anti-phase with the second switch, the capacitance to be measured and the reference capacitor being connected to a common summing node, the summing node being connected via rectifying means to processing means operable to control the relative outputs of the first and second voltage sources so as to minimize the charge at the summing node, and the processing means providing an output indicative of the value of the capacitance to be measured.
According to a third aspect of the present invention there is provided a fluid-gauging system, including a capacitive probe arranged to measure height of fluid and a measuring circuit according to the above first or second aspect of the present invention arranged to measure the value of the probe.
The system may include a capacitive cable connecting the probe in the system.
An aircraft fuel-gauging system according to the present invention, will now be described, by way of example, with reference to the accompanying drawings.